powerpc/pseries: Correct rtas_data_buf locking in dlpar code

The dlpar code can cause a deadlock to occur when making the RTAS
configure-connector call.  This occurs because we make kmalloc calls,
which can block, while parsing the rtas_data_buf and holding the
rtas_data_buf_lock.  This an cause issues if someone else attempts
to grab the rtas_data_bug_lock.

This patch alleviates this issue by copying the contents of the rtas_data_buf
to a local buffer before parsing.  This allows us to only hold the
rtas_data_buf_lock around the RTAS configure-connector calls.

Signed-off-by: Nathan Fontenot <nfont@austin.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>